Magnetic sensors come in various forms and have been in use for decades. Currently magnetic sensors are being integrated on an integrated circuit with other circuitry. Fluxgate sensors consist of magnetic cores wrapped with drive and sense wires. Hall Effect sensors are linear transducers commonly used to detect shaft rotations. Anisotropic magnetoresistance (AMR) sensors have varying resistance based on the angle of the magnetization. Giant magnetoresistance (GMR) sensing uses a quantum level effect observed in thin-film structures. Tunnel junction magnetoresistance sensors (TMR) operate based on the spin dependent tunneling of electron: through a thin, electrically insulating barrier layer. More recently, another type of sensor called extraordinary magnetoresistance (EMR) occurs in semiconductor-metal hybrid systems, where the resistance can change several orders of magnitude with the application of a magnetic field, thus providing a sensor.
The accuracy of magnetic sensors is dependent on various factors including temperature, process, aging, magnetic field orientation. The accuracy of the magnetic sensor can vary within the operational range, especially when the response is not linear with the magnetic field intensity. Traditionally, these variations are compensated for with various prior known approaches. The primary known method is to use a calibration procedure or process on the sensor. This calibration process may be a one-time calibration or the calibration can be periodic depending on the complexity of the calibration system, the ability to gain access to a sensor for calibration and the accuracy desired. Other known techniques include placing the sensor in close proximity to the source so that the predominant magnetic field is from the desired source. Another technique is to use stronger magnets to overcome fields from other sources, which increase cost and add their own variability from aging and temperature. In nearly all cases using prior known approaches, the magnetic sensor calibration is done while the sensor is off line, or the sensor may need to be physically moved. As more accuracy in magnetic sensing is required, the complexity and cost of the sensor calibration increases.
While each of the prior known approaches has provided some improvement, further improvements are still desirable. Methods and apparatus for calibrating the magnetic sensors accurately while the sensors are in use or are in place, and without significant added costs, are therefore needed.
A continuing need thus exists for methods and apparatus to calibrate magnetic sensors to increase the accuracy of the sensing operations, while maintaining or reducing system costs, and without increasing the size of the magnets or requiring removing the magnets or sensors from the system.